A capacitive touch screen is already widely used in various human-computer interaction devices. Nowadays, some capacitive touch screens can support finger touch and active capacitance stylus touch at the same time, and it requires the touch screen device can detect the contact of the finger and the active capacitance stylus with the touch screen.
In general, a touch screen device includes a touch screen and a touch screen controller, wherein the touch screen controller mainly detects the contact of the finger and the active capacitance stylus with the touch screen by the following structure: a reception unit (represented as RX), a transmission unit (represented as TX) and a signal processing unit, a mutual capacitor is connected between the reception unit and the transmission unit, and the touch screen may be taken as a part of the mutual capacitor. When detecting the contact of the finger with the touch screen, the transmission unit transmits a signal of certain frequency, and when the reception unit receives the signal, the signal processing unit may detect whether the finger contacts the touch screen according to an amplitude of the signal received by the reception unit, the amplitude of the received signal is related with the magnitude of the mutual capacitor, the magnitude of the mutual capacitor is changed when the finger contacts an intersection of RX and TX on the touch screen, i.e., the position of the mutual capacitor, which causes the amplitude of the received signal to be changed, therefore, the contact of the finger with the touch screen can be detected by detecting the amplitude of the received signal. When detecting the contact of the active capacitance stylus with the touch screen, the transmission unit is disabled and the active capacitance stylus is used as a signal transmission end, the signal processing unit detects whether the active capacitance stylus contacts the touch screen by detecting the amplitude of the signal transmitted from the active capacitance stylus and received by the reception unit.
During the above existing process of contact detection, the amplitude of the signal received by the reception unit is recovered after a demodulation process is performed on the signal received by the reception unit, and thereby the signal processing unit can detect the contact of the finger or the active capacitance stylus by determining the amplitude of the signal received by the reception unit. When detecting the active capacitance stylus, the demodulation signal may be asynchronous with the transmission signal since the signal transmission end is out of the touch screen device, which results in that the amplitude recovered after the demodulation process cannot correctly reflect the signal received by the reception unit.
The existing solution is to perform the demodulation after performing a phase synchronization between the demodulation signal and the transmission signal, and a correct phase can be found out by comparing demodulation signals of multiple phases with the transmission signal at the same time when performing the synchronization between the demodulation signal and the transmission signal; however, such way needs large numbers of hardware resources; another synchronization way is to adjust the phase of the demodulation signal step by step in a time-sharing manner, to make the demodulation signal synchronous with the transmission signal, so the hardware consumption can be reduced, but the synchronization time is often long which will affect the detection efficiency of the active capacitance stylus.